In the face of Lebanese opposition and U.S. reservations, Israel has put everyone on notice that it intends to begin pulling back some of its troops in Lebanon to new positions.

The consequences of the planned move are hard to predict, but it has put an already embattled Lebanese government in the embarrassing position of opposing recovery of sovereignty over one slice of land for fear of losing a larger chunk to a permanent Israeli occupation.

Nobody here will say exactly when Israel will begin what it calls a "redeployment" or just how far south the Israeli Army will move. But a senior government official kept emphasizing last week that "redeployment there will be" and "in the not-too-distant future."

The expectation is that it will start in August or early September, after Prime Minister Menachem Begin's trip to Washington for talks with President Reagan, and will be completed by November--events permitting.

While Israel is known to be considering three rivers--at distances ranging from 12 to 25 miles south of Beirut--as possible new lines for deployment of its forces, Israeli commanders have made it clear that they do not intend to relinquish some strategic positions in the Bekaa Valley, where Israeli and Syrian forces face each other.

The press here has been full of speculation about how, and to where, the Israeli Army intends to pull back. Judging from the public discussion as well as briefings by senior Army and government officials, a plan for a withdrawal in stages to the Awwali River just north of Sidon is already largely worked out.

This would leave Israel with the 27-mile "security zone" provided for under its agreement with Lebanon, except that Israeli rather than Lebanese Army troops would be responsible for keeping it secure.

The Army hopes the pullback will permit a cutback in call-ups of reservists and a substantial reduction in the total number of troops needed to police a smaller occupied territory. But spokesmen were unable to give any estimate of how big a saving in manpower the partial withdrawal would bring.

At the height of the war last summer, the Israeli Army had between 60,000 and 80,000 troops engaged in the invasion. Now it reportedly has less than 20,000 most of the time, although there are more at times for short-term training or buildups to counter Syrian threats.

Senior officials have been stressing that the pullback will be "in stages" and explaining that the idea is to see what happens in each phase before proceeding to the next. Israel's hope--and it does not seem to be much more than that right now--is that the Lebanese Army together with some combination of foreign troops, either from the 7,000-man U.N. force in Lebanon or the 4,200-man multinational force in Beirut, will fill the vacuum and prevent Syrian troops or Palestinian guerrillas from streaming back to the outskirts of the Lebanese capital.

Defense Minister Moshe Arens said Friday that he did not consider it a "mission impossible" for the Lebanese Army to take over and he expressed Israel's willingness to work out a schedule for withdrawal "that fits the capabilities" of the Lebanese.

"The pace of redeployment can be such that the Lebanese Army can handle it," he said.

He and other senior Israeli Cabinet ministers have been suggesting the withdrawal may even be position-by-position.

Officials emphasize that the Army does not intend to give up its key strategic positions in Lebanon's eastern Bekaa Valley, where Israeli and Syrian troops are face to face.

Chief among these posts is the stronghold atop Jebel Barouk, a mountain overlooking the Bekaa from the western side. There Israel has radar and electronic equipment capable of surveillance deep in Syria.

One Army source said Jebel Barouk gives Israel "an overview on the bedchamber" of Damascus and the Syrian government. It also is a commanding height from which to shell and interdict Syrian or other traffic on the Beirut-to-Damascus highway.

Another strategic "pressure point" Israel intends to keep, according to senior Army and government officials, is Yanta on the eastern side of the Bekaa. The town is less than 14 miles from Damascus, putting the Syrian capital within range of Israeli artillery there.

A third point the Israeli Army may keep is Ayn Zahlta, a village just a few miles south of the Damascus highway in the eastern Chouf Mountains from where the strategic road could also be shelled.

In the western part of Lebanon, Israel has been considering a pullback to any of three defense lines marked by rivers.

The first is the Damur River, which flows into the Mediterranean about 12 miles south of Beirut near the abandoned, war-devastated town of the same name. From the coast, the river runs northeast through the Rishmayya Valley to inside the Chouf region. It has its source in the mountains close to Jebel Barouk, thus providing a link to Israeli positions atop it.

To pull back to this line, Israel would have to give up its presence along the Beirut-to-Damascus highway from Baabda, on the eastern outskirts of the Lebanese capital, to Sofar, where the Israeli and Syrian lines are just a few hundred yards apart.

Still unclear is whether the Army and Foreign Ministry would keep their offices in a Saudi-owned villa in Baabda that overlooks the Lebanese Defense Ministry and the entire capital area. This center is regarded by Israel as the nucleus of an embassy and thus has considerable political importance.

But once the Army leaves the Damascus highway, there is no easy way for Israel to protect this center unless it keeps a corridor open to it.

Establishing a line at the Damur River would also mean the Army would have to evacuate its positions on the coastal road from the southern edge of Beirut's international airport to Damur.

From the Israeli viewpoint, the advantage of this line is that the Army would no longer have to police the worst areas of confrontation in the Chouf between Lebanon's Christian Phalangist and Druze militias.

While a Damur line could save Israeli manpower while still giving the Army access to the back door of the Chouf, it is unlikely to reduce Israeli casualties significantly, since most of them have been suffered elsewhere.

The second river line under discussion is the Awwali, 22 miles south of Beirut on the northern outskirts of Sidon, the "capital" of the south.

The Awwali also runs northeast from the coast toward Jebel Barouk. The most interesting political feature of this line is that it would run through Mokhtara, the home village of Walid Jumblatt, the Druze leader whose leftist and Syrian-backed militia is leading the fighting in the Chouf against the Phalangists.

A continuing military presence in Mokhtara would give the Israeli Army a pressure and contact point to deal directly with Jumblatt and thus keep a measure of Israeli influence over the Druze and over political and military activities in the Chouf.

The third river under discussion is the Sinik, just south of Sidon--a dry riverbed much of the year.

If it pulled back to this line Israel would give up control of Sidon and relieve itself of all the security and administrative problems its presence there entails. There have been many attacks on Israeli patrols in and around Sidon, so casualties would probably be reduced considerably if the Army leaves.

But Sidon, with a population of 200,000, is the main city of southern Lebanon, and Tyre, the next largest, is too far south to be a convenient replacement as the Army's main headquarters.

Israeli officers also fear that giving up Sidon would simply give Palestinian guerrillas and other anti-Israeli elements a haven to retreat to after attacks on Israeli patrols further to the south.

Thus it is regarded here now as unlikely that the Army will give up Sidon or set up its main defense line along the Sinik. Also, the Sinik does not provide an adequate landmark. The Awwali, by comparison, flows throughout the year, providing a natural barrier against infiltration.